Not Applicable
Not Applicable
This invention relates to the measurement of electric current using current transformers.
Most current monitoring systems for a-c (alternating-current) electric power systems utilize current transformers to provide input currents that are isolated from the electric power system conductors. A primary winding of a current transformer is connected in series with a current-carrying conductor while a secondary winding is magnetically coupled to the primary winding by a suitable magnetic core. A current is induced in the secondary winding that is proportional to the primary current. The secondary current is isolated from the primary current and is smaller than the primary current by the turns ratio of the primary and secondary windings. The primary winding frequently consists of only one turn, which is often just the current-carrying conductor installed through an opening in the middle of the current transformer magnetic core. The secondary winding usually consists of multiple turns wrapped around the magnetic core.
The accuracy of a current transformer is usually related to the coercive force of the magnetic core material (less is better), the cross sectional area of the magnetic core (bigger is better), the effective magnetic length of the magnetic core (shorter is better), any air gap in the magnetic core (less or none is better), and the xe2x80x9csquarenessxe2x80x9d of the magnetic core material hysteresis curve (squarer may be preferred if not operating near saturation, otherwise characteristics that are not square may be preferred). Split-core current transformer cores generally have hysteresis curves that are less square than standard current transformer cores due to the small air gaps inherent in the design of split-core current transformers.
In order for the secondary current generated by a current transformer to be an accurate representation of the primary current, the impedance of the circuit connected to the secondary winding must be kept low so that current can flow freely. The impedance of the secondary circuit is often called the xe2x80x9cburden.xe2x80x9d The burden generally includes all impedances in the loop through which the secondary current flows, including stray winding impedances, stray impedances of connecting conductors, and the impedances of any devices connected in the loop (such as current-sensing resistors and relay operating coils). In order for a current transformer to drive a secondary current through a non-zero burden, a voltage must be induced in the secondary winding. The induced voltage is proportional to secondary current and is proportional to the burden, in accordance with Ohm""s law. The induced voltage is induced in the secondary winding by a fluctuating induction level in the magnetic core (the instantaneous magnitude of induced voltage being proportional to the rate of change of magnetic flux). The fluctuating induction level is associated with an xe2x80x9cexciting currentxe2x80x9d in accordance with well-known electromagnetic principles. The exciting current is often understood to have a magnetizing component and a core loss component. The exciting current accounts for most of the error in the secondary current. Generally speaking, the accuracy of a current transformer is inversely related to the burden of the secondary circuit. A higher burden causes the current transformer to operate with greater induced voltage, thereby increasing the exciting current, thereby causing the secondary current to be a less accurate representation of the primary current.
As used herein, the phrase xe2x80x9cinduction levelxe2x80x9d is synonymous with xe2x80x9cmagnetic induction level,xe2x80x9d and refers to the amount of magnetic flux within a magnetic core.
To improve current transformer accuracy, it is common practice to try to minimize the burden of the secondary circuit. Two ways of minimizing the secondary circuit burden are:
(a) Utilize a current-sensing resistor with relatively small resistance, along with a sensitive amplifier.
(b) Utilize an active load to sense current (an active load can have an effective burden of virtually zero Ohms).
While both of these methods are able to reduce the burden of the current-sensing device, they have no effect on the burden of the rest of the secondary circuit (secondary winding impedances and the stray impedances of connecting conductors). It is therefore desirable to compensate for current transformer error even when current-sensing devices with low burdens are utilized.
Some specialized current transformers with multiple windings and/or multiple cores have been developed. Many of these transformers have greatly improved accuracy. However, most of these specialized transformers are prohibitively expensive for many applications.
Prior art methods for compensation of current transformer error include applying simple correction factors based on current magnitude. Correction factors may be used to correct both current magnitude and phase angle. While these methods may be suitable for some applications, they are often inadequate.
Some have developed compensation schemes based on digital signal processing techniques with some success. The goal has been to simulate the hysteresis properties of the magnetic core with mathematical models and calculate the exciting current error. However, these methods require significant processing capabilities to deal with the nonlinearities involved. Even when simplifying assumptions are made the computation task is difficult.
To overcome the limitations of the prior art, an analog means of accurately compensating for current transformer error is desirable. It is also desirable that a compensation circuit be suitable for use with standard current transformers, both new current transformers and those already in service.
The invention calculates the exciting current of a current transformer and adds this to the measured secondary current to produce a corrected signal. The exciting current is calculated by first calculating the current transformer induced voltage. The induced voltage is calculated from a parameter related to secondary current and known characteristics of loop impedances through which secondary current flows. The parameter related to secondary current may be the secondary current itself, or a voltage associated with secondary current flowing through an impedance.
The calculated induced voltage is then applied across an inductor having similar magnetic properties as the current transformer. By scaling the applied voltage properly, the current flowing in the inductor is approximately proportional to the exciting current of the current transformer. The current in the inductor is measured, and, by applying an appropriate constant of proportionality, the exciting current of the current transformer is determined. A corrected secondary current is then calculated by adding the calculated exciting current to the measured secondary current.